This invention relates to double-walled cup-shaped containers comprising an outer cup-shaped paper container and an inner thermoplastic synthetic resin container having a similar shape and a smaller thickness applied on at least the side wall thereof to the side wall of the paper container. The inner thermoplastic synthetic resin container is formed of a seamless continuous synthetic film or sheet. This invention also relates to the method for producing such double-walled containers which comprises the steps of placing a preformed paper cup having an adhesive applied to the inner surface thereof, in the cavity of a shaping die set, positioning a piece of of thermoplastic synthetic resin sheet or film on the top of the die set across the mouth of the paper cup, heating the resin sheet or film to render the sheet plastic and drawing the sheet or film under negative pressure into the cavity in the paper cup to cause the sheet or film to adhere to at least the inner surface of the side wall of the paper cup.
There have been proposed and practically employed a variety of double-walled paper cups for holding liquid products for drinking in the past. The prior art double-walled paper cup has been hithertofore produced by providing a preformed paper cup having an adhesive applied to the inner surface thereof, laminating a piece of synthetic resin sheet or film onto the adhesive-applied inner surface of the paper cup to form the inner layer of the paper cup and curling the mouth upper peripheral rim of the paper cup together with the laminated resin layer. The thus produced double-walled paper cup has the advantages that the curled mouth upper peripheral rim gives a pleasant appearance to the paper cup and that the curled construction of the cup mouth upper peripheral rim provides a substantial rigidity against deformation of the cup when gripped. However, although the paper material of the paper cup has the resin film laminated thereon, the liquid content tends to leak through the seam between the cup side wall and the resin film laminated to the paper material of the cup to make the paper cup soggy and weak. Especially, when the content of the paper cup is a liquid at elevated temperature, the content leaking and paper weakening problems become more serious.
Since the upper peripheral rim of the mouth of the paper cup has been curled together with the laminated resin film, although the external surface of the curled mouth upper peripheral rim of the paper cup is covered by the laminated synthetic resin sheet or film, a portion of the external surface of the cup side wall immediately below or contiguous to the curled mouth upper peripheral rim is exposed. Thus, when the user desires to drink the liquid content, his lips inevitably contact the exposed external surface portion of the cup side wall directly and this is not agreeable from the view point of hygienics. Although the prior art vacuum formed containers have the advantage that they can be produced at relatively less expense, since the container have a small thickness, the prior art vacuum formed containers have the disadvantage that they have a low shape holding ability insufficient to maintain their shape against deformation when gripped.
There have been also proposed paper containers for holding liquid products provided with a peeping window. The paper containers of the type have a transparent plastic film applied to the window area. Thus, the paper containers have the disadvantage that the production of the containers require much manual efforts to the degree that the containers are rendered not suitable for mass production system.